Volatile organic compounds (VOCs) are common indoor air pollutants that pose concerning health risks, with benzene being among one of the most toxic VOCs, classified by the World Health Organization (WHO) as a Group 1 carcinogen to humans. Benzene, which is an important feedstock for the production of fine chemicals like cyclohexane, can be difficult to capture from the air and separate from cyclohexane using conventional adsorbents, making both benzene pollution management and cyclohexane purification challenging. Researchers from the University of Manchester have now developed novel porous materials that improve the capture and separation of benzene, opening up new possibilities for the efficient and cost-effective management of benzene pollution and contamination.
The researchers designed and prepared two families of stable metal-organic frameworks (MOFs): UiO-66 and MFM-300. Both have precisely designed pore sizes and chemistries that enhance both their absorption affinity and selectivity for benzene, the authors wrote. To determine the ideal MOF designs for benzene capture and separation, the team tested four different UiO-66 materials and seven different MFM-300 materials incorporating different metals and pore sizes. They found that the inclusion of copper(II) sites in the UiO-66 MOF improved the adsorption of benzene, and the MFM-300 material demonstrated the highest selectivity for benzene versus cyclohexane when incorporating scandium and a pore size of 8.1.
At a pressure of 1.2 mbar and temperature of 298 K, the UiO-66-Cuii material adsorbed benzene at 3.92 mmol g-1, and the MFM-300(Sc) material achieved a high selectivity of 166 for separating a one-to-one volume mixture of benzene and cyclohexane. Adsorption of benzene using conventional materials such as activated carbon and zeolite requires much higher pressures (up to 127 mbar) to achieve maximum adsorption capacities, and due to structural disorder in these materials, benzene uptake is significantly reduced at lower pressures. For separation of benzene and cyclohexane, highly expensive and energy-intensive distillation processes are required for purification purposes, due to the nearly identical molecular sizes and boiling points of the two chemicals. With the two new MOF materials developed by the University of Manchester team, capture and separation of benzene can be made more efficient, cost-effective and sustainable. This research was published in Chem.
“The crystalline nature of MOF materials enables the direct visualisation of the host-guest chemistry at the atomic scale using advanced diffraction and spectroscopic techniques,” said co-lead author Sihai Yang. “Such fundamental understanding of the structure-property relationships is crucial to the design of new sorbent materials showing improved performance in benzene capture.”